Tuesday, July 01, 2014

More Interesting than Watching Nail Polish Dry

Some salon manicures require you to dry your freshly painted nails under a UV light. But why only certain types of manicures and why UV? Why don't you always dry your nails with a UV light? And when you do paint your nails at home, why does cooling wet nails help them dry faster?

Image Credit: Photo (c) 2010. J. Ronald Lee 
Nail polish fundamentally consists of four things: solvents, polymers, pigment, and plasticizers. The plasticizers increase the flexibility of the polish, and pigment adds color. The polymers strengthen the polish, make it easy to spread, and help it stick to the nail. The solvents keep everything mixed together until they evaporate, leaving behind a smooth nail polish.

When painting nails at home, most people just wait around for the nail polish to dry. The older the polish, the longer it takes. The internet is full of tips on how to dry nail polish faster, like setting a hair dryer to 'cool,' sticking your hands in a freezer, using compressed air, and submerging nails in ice water. A lot of these tips focus on lowering the temperature of the nail polish, and the answer why has to do with thermal physics.

There is a certain pressure and temperature at which gas can coexist with a liquid, called the vapor pressure. When the liquid nail polish and the air around your nails are in this state, the polish is evaporating at the same rate that the air around the nail condenses. At room temperature, the evaporation rate is a little faster, the solvents evaporate slowly out of your nail polish, and your nail dries at the 'usual' rate. When you cool the nail, you're lowering the vapor pressure of the solvents in the nail polish and allowing them to evaporate faster, drying your nail quicker.

Image Credit: Thinkstock

Then why do you have to finish your nails under a UV light sometimes? Gel manicures are a specific kind of manicure that require UV lights (and sometimes LED lights) to cure the polish. Gel manicures apply a thick gel over the nail (and sometimes nail extensions). These gels also include a photo-initiator, a component that decomposes when exposed to light. These decomposed molecules have an unpaired valence electron that kick-starts the curing process of the gel. If the gel in the manicure is too thick, then sometimes the UV light can't penetrate to the lower layers to cure it. Some gel manicures use LED light to the same end, but the UV version is more common.

A few nail polishes do recommend drying under an incandescent light bulb. Those nail polishes require a small amount of heat (which is why florescent bulbs won't work as a replacement) to set the nail polish. Most nail polish that you buy at the drug store or retail store doesn't require heat or UV light to dry. The next time you paint your nails, try drying them by cooling them down. At least when your friend asks why you have your hands in a bowl of ice water, you can blind them with science.

Special thanks to Tom Vichroski of CRDR Consulting, Inc. for his cosmetic chemistry expertise. 

5 comments:

  1. Nice article and sound arguments. Just shared this post with a colleague…

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  2. Your vapor pressure statement is backwards. Higher temperature creates a higher vapor pressure, meaning faster evaporation. Just FYI...

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    1. That is correct.

      It is an established fact that ordinary (i.e. acetone-based) nail polishes harden more quickly in water around 35°F (2°C) than in air which is around 70-75°F (20-23°C).

      However, this is not necessarily a complete process. What is happening is the top few molecules of the nail polish harden quicker but the ones underneath remain soft. There's a skin over a layer of polish; without taking care, it is still able to be dented and smeared because it's soft.
      It's also important to note that "hardening" is not the same thing as "drying". Nail polish does not dry in ice-cold water. The only way to dry it quicker is to somehow increase the vapor pressure of the acetone to make it leave the nail polish in a shorter time.
      The vapor pressure of acetone increases logarithmically as the temperature increases. For practical purposes, the most significant range of temperatures are between 45-120°F (about 8-50°C), where the vapor pressure range is between about 100 and 750mm.

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    2. PS: If the nail polish is applied in a "really thin" layer, the entire layer may harden. But I suspect you'd have to apply it so thinly that several coats would be needed before it was as thick as "normally" applied polish.

      To my way of thinking, using ice water to obtain a harder skin is something that might make people feel less prone to accidents provided they continue to take as much care as before to dry the polish before using the fingers for anything much. At best, it will stop dust in the air settling on the polish and spoiling the surface.

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    3. Your paragraph about vapor pressure is completely wrong and makes no sense. Air does not condense under any of the conditions where one would be painting one's nails.

      And as the previous posters mentioned, cooling lowers the vapor pressure which reduces the rate of evaporation.

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